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Home arrow VMCA Newsletters arrow Volume 4, Number 1 - January 1, 2007 arrow Battery Coil Ignition: The Kettering System
Battery Coil Ignition: The Kettering System PDF Print E-mail
Written by BMWVMCA NEWS   
Wednesday, 06 August 2008


Battery Coil Ignition: The Kettering System

The following comes from ‘Mustang Sally’s Land Rover Pages’

The website owner authorizes reprint privileges to non-profit organizations.


An internal combustion engine requires a source of energy to ignite the fuel-air mixture in the cylinder at the right time. In a diesel engine, the rapid compression of the inlet charge raises its temperature to the point of self-ignition, hence the alternative name, compression-ignition engine. This method needs high compression ratios, typically 22:1 and a heavily built engine to withstand the forces involved.

The more lightly built petrol engine has a typical compression ratio of 8 or 9:1 and the compressed charge does not get hot enough. Some outside help is needed to initiate combustion. Various ideas were tried in the very early days, such as the hot tube system which had a piece of metal protruding into the combustion chamber and brought to red heat by an external blow-lamp arrangement.

An electric spark plug works much better. The problem is (a) turning six or twelve volts from a battery into the 20,000+ volts needed to jump across the 1mm spark plug gap and (b) persuading the spark to occur at just the right moment of the pistons travel.

The standard Kettering circuit fulfils these requirements. As illustrated for a four-cylinder engine, it consists of a battery, an ignition coil, a capacitor, a pair of points arranged as a contact breaker, an ignition distributor and at least one spark plug per cylinder.




The ignition coil commonly consists of a secondary winding of many thousands of turns of fine wire wound around a laminated iron core. Wrapped over the outside of the secondary is a primary winding of a few hundred turns of thicker wire. The whole lot is housed in a metal can, usually filled with dielectric oil to aid heat dissipation. The secondary has a resistance of some thousands of ohms. The primary has a resistance of between one half and several ohms depending on the design. The lower ends of the two coils are joined inside the can and are brought out to the negative terminal on the coil top. The upper end of the primary comes out at the positive terminal while the upper end of the secondary is connected to a well insulated socket for the high voltage lead to the distributor.




The ignition points are housed within the distributor base and are opened by a cam on the distributor shaft which turns at half engine speed. Closure is effected by a spring. The distributor cam has one lobe for each cylinder and is positioned on the shaft so that the points open near the start of each power stroke. The moving arm of the points set is connected to the negative terminal of the ignition coil ( in a negative earthed electrical system) by an insulated lead passing through the distributor base.

When the points are closed, a current of up to ten amps flows from the battery, through the primary winding and away to ground via the points. The current flow produces a magnetic field around the coil windings which is enhanced and concentrated by the iron core.

When the ignition points open, the primary current suddenly stops and the magnetic field collapses. The rapidly changing magnetic field induces a sharp voltage spike in the primary winding. This voltage spike is several hundred volts negative with respect to the +12volt applied to the battery side of the primary and because the coil is acting as a transformer, a pulse of up to 20,000 volts appears across the secondary winding. The voltage required to fire the sparkplug determines the maximum voltage which actually appears on the High Tension terminal and this can range from 5000 to over 15000 volts, depending on the plug gaps and the conditions inside the combustion chamber. If a sparkplug lead happens to be disconnected, the secondary voltage can rise to the maximum possible figure and cause unnecessary strain on the insulation of the coil and the high voltage leads.

The voltage spike in the primary will strike an energy wasting arc across the just-opened points unless something is done about it. This is one function of the capacitor connected across the points, an electrical device which has the ability to accept an inflow of current until it is fully charged. Coil current continues to flow briefly into the uncharged capacitor while the points move apart to the point where an arc can't be struck by the few hundred volts of the primary spike. Once the capacitor is fully charged, the current flow stops and the magnetic field can collapse. The voltage spike in the primary rapidly rises to its peak value, inducing the multi-thousands of volt pulse in the secondary. Without the capacitor, the points will be rapidly eroded  and the collapsing magnetic field will expend a good part of its energy as a few dozen volts maintaining the unwanted arc across the points.

The High Tension voltage pulses induced in the secondary are conducted from the top of the coil back to the distributor cap centre post, from where the rotor arm feeds it to the correct spark plug lead at the appropriate time.

At ordinary engine operating speeds, the points open and close a couple of hundred times per second, the exact number depending on the number of cylinders and the engine RPM. The points need to be closed for a appreciable time in order to build up the maximum magnetic flux in the ignition coil core. The period of points closure is specified by the ignition system designer and is typically expressed as degrees of distributor rotation. In a four cylinder engine, the angle between each ignition cam lobe is 90 degrees and the period of points closure or "dwell" is usually a bit over 45 degrees of distributor rotation. In a six cylinder engine, the lobes are 60 degrees apart and the dwell time is 30 to 35 degrees. The dwell is adjusted by setting the points gap to a specified distance at maximum opening. A narrower gap gives more dwell and a wider gap gives less. Taking it to extremes, excessive dwell means that the points close too soon after opening, cutting off the magnetic field collapse before it delivers all its energy. Too little dwell gives the magnetic flux insufficient time to build up to the maximum. Both conditions give a a weak spark which gets even weaker as the engine RPM rises and produces misfiring at normal operating speeds.

Advantages of the points/coil system.

- Simplicity and cheapness.

- Plenty of spark energy for starting and at low engine speeds.

- Ease of maintenance in the field with widely available spare parts.

Disadvantages

- Spark energy is reduced greatly as engine revs rise. The problem becomes even worse in multicylinder installations having smaller points dwell times.

-  High maintenance. The points assembly starts to wear and go out of adjustment as soon as it enters service and needs regular inspection, resetting and eventual replacement if the engine is to remain in proper tune.

Defects of the Kettering Ignition System.

- The ignition cam which rotates with the distributor shaft bears on a  heel or rubbing block attached to the moving point arm. The rubbing block must be softer than steel to avoid scoring the metal of the cam and the only lubrication is the thin smear of grease applied when the points are serviced. Various synthetic materials can be used for the block but it still wears down over some thousands of miles, decreasing the points gap at maximum opening. This retards the ignition timing which saps engine performance. In extreme cases the points may close almost immediately after opening which quenches the spark before it has a chance to ignite the fuel mixture properly.

- The rubbing block will eventually wear the ignition cam over high mileages, possibly leading to inconsistent points timing and lift between cylinders and accelerated wear of the block on new point sets. The sideways pressure of the rubbing block also encourages wear of the distributor shaft bearings. Wobbly bearings means a wobbly ignition cam and inconsistent points operation.

- At high engine RPM, the moving point arm accelerates faster under spring pressure when closing and will bounce a little when the moving point meets the fixed point. This interferes with the flow of primary current in the coil and reduces the energy available for the next spark. The fitting of "high performance points" with a stronger spring alleviates the problem but increases the rubbing block wear and the sideways pressure on the distributor bearings.

- The actual time the points spend closed decreases as engine RPM rises. In a typical four cylinder ignition, the points are closed about 50% of the time. With a typical coil, the time needed for the coil current to reach its maximum value (for maximum magnetic flux and maximum spark energy) is around 15 milliseconds. If sparks are needed less than 30 milliseconds apart, the spark energy will be less than the maximum possible. Sparks spaced 30 milliseconds apart translates to a spark rate of 33 sparks per second, which is only 990 RPM for a four cylinder engine. That engine will be barely off idle before the spark energy begins to fade away.

- The contact faces of the points are made of a tungsten alloy to combat the erosion induced by switching several amps thousands of times a minute. The contacts will wear eventually, usually by transferring metal from one face to another, leaving a pit on one side and a pip on the other. It is impossible to gap points in that condition with a feeler gauge. Filing the pip can extend the life of the assembly but some skill is needed to ensure that the two faces remain square to each other.

- Most coil makers offer a "sports coil" replacement for their standard coil. These have thicker wire in the primary winding so that heavier coil current can flow giving higher energy. These coils go some way towards alleviating the drop off of energy at higher RPM. The trade-off is that the coils run hotter and the heavier current wears out the points contact material faster. (Sally’s AU Site)

- The disruptive discharge Tesla coil is an early predecessor of the "ignition coil" in the ignition system. Tesla also gained U.S. Patent 609250 , "Electrical Igniter for Gas Engines", on August 16, 1898. It used the principles of the ignition coil used today in automobiles. A. Atwater Kent, in 1921, patented the modern form of the ignition coil. (Wikipedia)

- Kent was always interested in automobiles and, particularly, in the means of igniting internal combustion engines. He patented the contactor, a breaker point mechanism, and the distributor to enable the use of a single coil. Income from his ignition systems enabled Kent to enter the radio business with a fully equipped manufacturing facility, a national service organization and an appealing concept, the open Set.  (Wikipedia)

- Dangers: Large Tesla Coils and Magnifiers can deliver dangerous levels of high frequency current, and they can also develop significantly higher voltages (often 250,000–500,000 volts, or more). Because of the higher voltages, large systems can deliver higher energy, potentially lethal, repetitive high voltage capacitor discharges from their top terminals. Doubling the output voltage quadruples the electrostatic energy stored in a given top terminal capacitance. If an unwary experimenter accidentally places himself in path of the high voltage capacitor discharge to ground, the high current electric shock can cause involuntary spasms of major muscle groups, and may induce life-threatening ventricular fibrillation and cardiac arrest. Even lower power vacuum tube or solid state Tesla Coils can deliver RF currents that are capable of causing temporary internal tissue, nerve, or joint damage through Joule heating. In addition, an RF arc can carbonize flesh, causing a painful and dangerous bone-deep RF burn that may take months to heal. Because of these risks, knowledgeable experimenters avoid contact with streamers from all but the smallest systems. Professionals usually use other means of protection such as a Faraday cage or a chainmail suit to prevent dangerous currents from entering their body. (Wikipedia)

Last Updated ( Monday, 08 June 2009 )
 
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